At such double chainstitch sewing machines the so-called looper thread and the so-called needle thread will be processed in the workpiece to form a thread-chain, wherein a thread-chain of such type will also be built if the workpiece has not yet reached the stitching needle or if the workpiece has already passed the stitching needle.
For the separation of threads it is known from U.S. Pat. No. 3,159,124 to arrange in the area of stitch formation a suction tube as to suck in the threads, respectively the thread-chain, and to arrange in the suction tube a heating wire in order to thermically melt off the sucked in thread. This known arrangement has the disadvantage that the heating wire will be surrounded by the sucked air and thus cooled. Due to this condition it is necessary to provide for higher heating power. Moreover, the surface temperature of the heating wire depends on the individual air flow rate of the suction tube. Danger exists in a heating wire arranged in this manner in that the sucked in thread tends too get caught or does not have a sufficiently long enough contact with the heating wire to guarantee a complete separation at a definite position.
From British patent specification No. 1,093,275 it is known to provide a cutting plate heatable by a heating element at the front end of a suction tube. This heating plate does not render possible the separation of the threads at an exactly defined position, and it requires a relative high heating power.